Examples of tape media that, e.g. an LTO tape drive and an IBM™ enterprise tape drive TS1140 use include general tape media, called R/W media, and also WORM media in which data, once written, cannot be erased, in other words, which is used like a CD-R.
The tape drives have the function of detecting tampering of data written to a WORM medium, committed by a malicious third person, when reading the data.
IBM™ has developed a file system that operates on a tape device, named a linear tape file system (LTFS).
Some versions can be downloaded from the Web without charge, and some other versions are priced and are on the market on a chargeable basis.
The LTFS is an open standard system; a plurality of companies do business using products based on the LTFS standard.
When a tape medium is to be used in the LTFS, the tape medium is divided into two partitions called an index partition (IP) and a data partition (DP).
When a user writes a new file on a tape medium using the LTFS, the data partition (DP) is mainly used as an area for storing a file main body, and the index partition (IP) is mainly used as an area for storing an index, to which metadata called an index is written.
The index has metadata of the file, such as file name and file creation date.
The index partition (IP) is a partition to which the latest index is typically written.
The data partition (DP) is a partition to which the file main body and also the history of the index are typically written.
Assuming that the present LTFS is used on WORM media, files recorded on the WORM media (including a directory, ditto for below) can be tampered or deleted, although data recorded on the WORM media cannot be deleted.
This is because tampering or deletion of files in the LTFS is performed by additionally writing a file main body or metadata without involving deletion of data recorded on the WORM media.
Example related art for achieving the WORM file system is as follows:
Patent Literature 1 discloses a technique for a WORM file system that rejects tampering or deletion of not only a bit array stored in a storage but also a file itself on a WORM medium.
The title is “Write-once-read-many storage system and method for implementing the same”.
Patent Literature 1 discloses a system in which an attribute, WORM bit, is prepared to implement WORM for each file, and in which update/deletion of files is enabled before the WORM bit is turned on, and is rejected after the WORM bit is turned on.
The capacity of tape media is increasing year by year; for example, the capacity of JC tape media that TS1140 uses is 4 TB.
For using the LTFS, there is a need for writing a large amount of files with an average size of about 10 KB to a tape medium.
In particular, for storing a large amount of files in a tape medium, if bits for implementing WORM are to be prepared for each file, the time required to update the WORM bits and the volume of the WORM bits in the metadata cannot be disregarded.
If files with a file size of 10 KB are stored in a tape medium with a capacity of 4 TB, the number of files per tape medium is four hundred million. Assuming that the length of an attribute (<worm>0</worm>), including line feed code, for implementing WORM stored in XML is 15 byte, the volume of WORM bits in metadata when four hundred million files are present is 6 GB.
Furthermore, it takes 24 seconds to read or write the information from the tape medium at 250 MB/sec.
Patent Literature 2 discloses implementation of WORM for files and directories by explicitly executing a command for WORM, which differs from the present invention.